A rechargeable nickel-cadmium battery stores electrical energy and makes this stored electrical energy available during discharge. The energy discharge performance characteristic may vary from one battery to another due to application-related factors, differences in the design of the cell, differences in its internal construction, and the condition of actual use of the battery. In all these cases of variable discharge performance, the nickel-cadmium battery maintains its charge (load-carrying capacity) until very near the end of its useful charge period, then discharges rapidly to a completely discharged level within a few minutes. Detecting the point at which the battery is about to discharge completely in all of the varying situations presents a formidable but important challenge, as complete discharge of a battery greatly shortens its useful life.
Traditional approaches to monitoring the rechargeable battery-energy discharge have been to either use fixed voltage levels to indicate the battery's useful charge period, or to use the rate of change of voltage (slope) as an indicator of the rate of discharge of energy. In applications, such as computer systems, these traditional approaches to battery power monitoring become impractical and unreliable because a rapidly declining rate of change of voltage often occurs long before a battery's useful output is about to be dissipated.
In particular, large negative slopes, which simulate the negative slope values that occur shortly before complete battery discharge, can be caused by 1) varying load conditions on the battery, for example, due to disk accesses, 2) cell dropout, 3) startup voltage depression, and 4) overcharge voltage depression.
A reliable battery power management system should be capable of distinguishing among the various causes of large battery discharge negative slopes and effect warnings and shutdown only in response to imminent complete battery discharge.